Applying digital methods to the control of systems bears the promise of creating new features, improving performance, providing greater product flexibility, and providing a lower cost. System operating characteristics dictated by a stored program, rather than the parameters of a set of discrete components, can result in cost and space savings as well as capacity for real time adaptation of those characteristics, greater sophistication in control algorithms and the ability to generate, store and recall valuable real-time functional data.
However, digital feedback control requires high resolution and high speed. These requirements have limited the adoption of digital control in many fields. The advent of low cost logic has it made possible the application of digital control techniques to cost sensitive fields. As the cost of digital logic decreases, new opportunities arise.
A typical digitally controlled feedback system has an analog to digital converter, digital loop compensator, power device driver, and an external system to be controlled. An example of a system in which application of digital control can improve performance or lower cost is the switching power supply or DC-to-DC converter. (However, many other systems would also benefit from application of digital control.)
It is very desirable to minimize the cost, size and power dissipation of a low-cost off-line switching power supply for low power applications, such as recharging cells and batteries used in portable consumer appliances, such as entertainment units, personal digital assistants, and cells phones, for example.
A PWM switched power supply requires a variable pulse width that is controlled by an error signal derived by comparing actual output voltage to a precise reference voltage. The pulse width of the switching interval must also be constrained to be within a minimum and maximum duration. These constraints are imposed for correct PWM power supply or motor driver operation.
An example of a digitally controlled system is shown in FIG. 1. In the example shown in FIG. 1, the system is a simple buck Switching power supply. The fundamental components are the same for any Switching power supply. The sample system shown in FIG. 1 includes three major components: a compensator preceded by an ADC, PWM and power switches, and passive LC network.
Most power management design is based on simple, continuous compensation using frequency domain analysis. Bode analysis is frequently used as the design technique of choice.
More modern techniques such as modeling of the converter in discrete time and using pole placement or optimization techniques to set the gains are not usually considered. Recently developed digital power management chips use the digital equivalent of analog continuous time designs. The design procedure starts with an analog prototype which is discretized and implemented in hardware.